1. Field of the Invention
The present invention relates to an image forming apparatus which can change an image forming order and a control method of the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus for forming a color image onto a transfer member by overlaying toner images of a plurality of colors, there is a method whereby electrostatic latent images are formed onto an image bearing member, sequentially developed by toner, and transferred onto the transfer member every development, and the color toner images of a plurality of colors are overlaid. There is also a method whereby a plurality of toner images are overlaid onto an intermediate transfer member and transferred onto the transfer member in a lump. However, in both of the methods, such a phenomenon that the toner transferred onto the transfer member or the intermediate transfer member is transported onto a photosensitive member (hereinafter, referred to as a re-transfer phenomenon) occurs.
The re-transfer phenomenon occurs by the following reason: the toner whose charged charge amount is small among the transferred toner or the toner which has been charged to an opposite charge by an influence of a discharge phenomenon or a transfer current at an image transfer position (also referred to as a transfer nip) is transported onto the photosensitive member by an electrostatic force, a van der Waals force, and the like.
An adverse influence on the image by the re-transfer phenomenon is enhanced with an increase in use amount (the number of prints, a driving time of a developer holding member, etc.) of a developing apparatus. It is presumed that this is because when the use amount of the developing apparatus increases, the charged charge amount of the toner decreases by a change (particularly, deterioration) in surface state or the like of each of a developing roller, a developing blade, and the toner.
An adverse influence on the image by the re-transfer phenomenon is caused so that the external additive agent may be buried under the toner, and the external additive agent may separate from the toner moreover.
For example, in the conventional image forming apparatus, a technique for reducing the adverse influence on the image exists.
For example, there is a method whereby concentration data of all pixels of one page of an original document is accumulated from original document image data of every reproduction color (every color of yellow, magenta, cyan, and black) and images are sequentially formed in ascending order from the color of the small accumulation value (for example, refer to Japanese Patent Application Laid-Open No. 2000-330354). According to such a method, the influence of the re-transfer phenomenon can be reduced. Its reasons will now be described. For example, the toner which has first been transferred from the photosensitive member onto the transfer member or a recording medium has three opportunities for re-transfer to occur. The toner which has finally been transferred from the photosensitive member onto the transfer member or the recording medium does not have the opportunity for the re-transfer phenomenon to occur. That is, the number of opportunities for the re-transfer phenomenon to occur is larger when the image forming order is earlier. Therefore, by forming the images in order from the image in which an amount of toner that is transferred onto the transfer member or the recording medium is small, the influence by the re-transfer phenomenon can be reduced.
There has also been known a method whereby by removing the charges on the surface of the photosensitive member before the photosensitive member enters the transfer nip, a potential difference between the photosensitive member surface and an intermediate transfer belt or the transfer member is decreased, the transfer current and the discharge amount in the transfer nip are reduced, and an amount of reversely-charged toner is decreased, thereby preventing the re-transfer. For example, refer to Japanese Patent Application Laid-Open No. H05-165383.
However, in the related art, there are the following problems. According to the invention disclosed in Japanese Patent Application Laid-Open No. 2000-330354, the images are sequentially formed in ascending order from the color of the small accumulation value of the concentration data of all pixels. However, if the use amount of the developing apparatus which first forms the image is large, there is a case of occurrence of the adverse influence on the image by the re-transfer. The foregoing adverse influence on the image is typical when an image in which the image data has locally been concentrated is printed.
According to Japanese Patent Application Laid-Open No. H05-165383, although the re-transfer phenomenon can be remarkably reduced by exposure of the photosensitive member before the transfer, another adverse influence on the image such as scattering caused by the exposure before the transfer, deterioration in dot reproducibility, or the like occurs.
The invention is made in consideration of the above problems and it is an object of the invention to provide an image forming apparatus in which by changing image forming order according to a use situation of a developing device, a re-transfer phenomenon of toner is reduced without causing scattering and deterioration in dot reproducibility and to provide a control method of the image forming apparatus.